Geology and Metamorphism of the Eastern Mesabi Iron Range

INTRODUCTION The field trip today will concentrate mostly on the metamorphic effects on the Biwabik Iron Formation, at the Peter Mitchell mine of eastern Mesabi Iron Range, caused by intrusion of the nearby Mesoproterozoic Duluth Complex (1.1 Ga). The stratigraphy of the iron-formation that is used at this mine, and nearby mines, was mainly developed by Gundersen and Schwartz (1962) and is depicted in Figure 3-1. This stratigraphy has since been modified by Bonnichsen (1968) in nearby Dunka Pit, and by mine personnel at the nearby Erie/LTV mine (now referred to as the Cliffs Erie site in Figure 3-1). While the stratigraphy is important as an ore control it will take a backseat in this portion of the trip in order to more fully concentrate on metamorphism of the iron-formation. Metamorphic effects in easternmost Mesabi Iron Range have greatly modified the mineralogy of the iron-formation through progressive devolatilization reactions, and near obliteration of primary textures, especially as the contact of the Duluth Complex is approached. Recrystallization and replacement of primary iron minerals and granules during diagenesis and metamorphism have been extensive, and consisted of a number of discrete events. The mineralogy of the Eastern Mesabi Range is reviewed in detail by Gundersen and Schwartz (1962) and that of the Dunka Pit area by Bonnichsen (1975). The following discussion serves to compliment the work of these researchers with work performed to date by Cliffs Natural Resources geologists in the area of Northshore Mining’s Peter Mitchell Mine. In contrast to the iron-formation of the Western and Central Mesabi Range, which is dominated by very low greenschist metamorphic facies assemblages, rocks of the eastern Mesabi have been subjected to middle greenschist through upper amphibolite facies metamorphic conditions. Hornblende and pyroxene hornfels facies are also observed at the contact of iron-formation with the Duluth Complex. Progressive effects of contact metamorphism toward the contact of the Duluth Complex have produced distinct mineral assemblages that document changing metamorphic conditions and concomitant changes in mineralogy. These changes are readily apparent in the highly reactive mineral assemblages of the Biwabik Iron Formation.